Spatial division multiplexing (SDM) in form of mode division multiplexing (MDM) is one solution to keep up with the increasing data traffic demand . MDM is based on using individual modes of a few-mode or multi-mode fiber as individual transmission channels and requires the addressing of individual modes at the input and output of the multimode fiber link. Here, among others, mode-selective fused fiber couplers based on evanescent coupling can be applied for addressing individual modes of fiber optic transmission links. In this case, light coupling occurs when the mode propagation constants are equal and this can be achieved e.g. by applying grating structures and, thus, compensate for the mismatch of the propagation constants of the modes to be coupled .
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